1. Field of the Invention
The present invention relates to optical spectroscopy. More particularly, the present invention relates to an optical method and apparatus for automatically providing a controlled and displayed power at a desired sample plane using calibrated wavelengths and optical characteristics of a system.
2. Discussion of the Related Art
When illuminating light, such as a laser beam, is incident upon a sample material, molecular bonds in the material can be excited by the incident light and can emit radiation which can be detected as scattered light. The Rayleigh component of the scattered light corresponds to the light emitted when the molecule relaxes from the excited state to the ground state. Infrequently, the molecule relaxes to a different vibrational or rotational level in the ground state. This produces Raman scattering components at Stokes and Anti-Stokes frequencies. A sample composed of multiple molecular species will produce a spectrum of such Raman scattering. The Raman scattering components can be detected and analyzed to help determine the composition of the sample.
Various apparatus have been developed for analyzing Raman spectra including Raman microscopes in which a very small area on a sample can be analyzed to determine characteristics of the composition of the sample at that area. In a typical Raman microscope, narrow band or monochromatic illuminating light, such as from a laser, is passed along a beam path through the objective lens of the microscope where it is focused at a focal point on a specimen. The intensity of the induced Raman scattered radiation is proportional to the applied optical intensity produced by the focused beam at the sample, wherein such induced radiation from the sample is collected by the microscope objective and is passed back on a beam path to a spectrograph, which typically separates the Raman scattering radiation by wavelength and detects it. Optical elements are typically included in the excitation beam path and the returning Raman radiation beam path to separate the excitation light from the Raman scattering light and to filter out the Rayleigh light from the beam directed to the spectrograph.
Available commercial instruments do not directly measure such focused illumination optical power at the plane of interest when characterizing a sample in a spectrometer. Such a measurement often requires removing the positioned sample either prior to and/or after optical interrogation in order to insert a detector at the optical plane of interest. Such a procedure captures the intensity of the beam that was desired to be applied during the actual measurement procedure.
To overcome removing the positioned sample, it is conventional to estimate optical power at the illumination plane of interest by simply displaying a percentage of the total laser power. This has the disadvantage that the actual laser power is unknown and has the consequence of the applied power changing as the power level declines through the service life of the laser.
Another method that has been utilized to resolve the aforementioned problem includes configuring an internal power meter, such as a beam-splitter and detector, with a single point calibration factor to estimate power at the sample. However, this has the disadvantage that different desired application wavelengths when coupled with components and accessories whose optical properties vary due to inherent and induced wavelength dependencies, may and often do affect the actual applied power at the desired sample plane, which is unaccounted for by a single point calibration.
Accordingly, a novel Raman spectrograph capable of supporting multiple laser wavelengths that includes interchangeable optical components with the ability to display and control optical power at the sample plane is needed. The present invention is directed to such a need.